Fairly sure Steam was mandatory for Orange Box and was also probably the first (or thereabouts) time Steam did the key sharing thing - getting the Orange Box bu

Fairly sure Steam was mandatory for Orange Box and was also probably the first (or thereabouts) time Steam did the key sharing thing - getting the Orange Box bundle would give you extra keys for the bits you already owned. This model isn't really how the bulk of bundles sold on Steam work now, though, for whatever reason - bundles tend to just get steep discounts on sale with possible extra discounts for content you already have.